The present invention relates to methods of forming block polymers for directed self-assembly applications, more specifically methods of making high-chi (χ) block polymers prepared by ring opening of cyclic carbonyl monomers.
Block copolymers (BCPs) find many applications in solution, bulk and thin films. Thin film applications of BCPs are particularly attractive for nanolithography and patterning due to the ability of some BCPs to form periodic self-assembled structures ranging in feature size from 5 nm to 50 nm. The thin-film self-assembly property of BCPs can be utilized with existing photolithographic techniques to provide a unique approach to long range order for semiconductor applications. This approach, called directed self-assembly (DSA) of block copolymers, promises to extend the patterning capabilities of conventional lithography.
BCPs for directed self-assembly (DSA) applications comprise two or three polymer blocks that can phase separate into ordered nanoscopic arrays of spheres, cylinders, gyroids, and lamellae. The ability of a BCP to phase separate depends on the Flory Huggins interaction parameter (χ). PS-b-PMMA is the most widely used block copolymer for DSA. However, the minimum half-pitch of PS-b-PMMA is limited to about 10 nm because of lower interaction and interaction parameter (χ) between PS and PMMA. To enable further feature miniaturization, a block copolymer with higher interaction parameter between two blocks (higher chi) is highly desirable.
Several block copolymers having higher interaction parameter between the two blocks have been studied to obtain smaller feature sizes. Of particular interest are block copolymers comprising a block derived from ring opening of a cyclic carbonyl monomer from a reactive end-group on the first polymer block. Block copolymers derived by ring opening polymerization (ROP) of cyclic monomers (e.g., lactides, lactams, lactones, ethylene oxide) have been used to generate sub-10 nm feature size for patterning applications.
In the case of ring opening polymerizations (ROP) of cyclic carbonyl monomers, several side reactions can occur. For example, ROP of cyclic ester monomers (i.e., lactides and/or lactones) can include transesterification reactions and/or end group backbiting reactions (Macromolecules, 1998, 31, 2114) that can degrade the polyester backbone, broaden the molecular weight distribution of the block copolymer, and/or form residual free polyester chains and/or cyclic polyester oligomers. Similar side reactions can occur in ring opening polymerization of cyclic carbonates that can significantly broaden the molecular weight distribution and ultimately lead to bimodal GPC traces (Macromolecules, 2011, 44 (7), pp 2084-2091). The presence of moisture or other nucleophilic impurities either in the cyclic carbonyl monomer, reaction solvent, catalyst, and/or the initiator can also form ring opened oligomers or ROP homopolymers in addition to the desired block copolymers (Macromolecules 2010, 43, 8828-8835). Oftentimes the cyclic oligomers or homopolymers formed as side reactions cannot be easily separated from the block copolymers, resulting in poor thin-film self-assembly behavior. For good thin-film self-assembly, block copolymers that are free of homopolymer and that have low polydispersity index (PDI) are preferred.
Thus, there exists a need for methods of forming ROP based block copolymers for self-assembly applications that minimize the amount of homopolymer, cyclic oligomer and other byproducts derived from ring opening of cyclic carbonyl monomer.